If you're in the market for an HDTV, you've probably heard a lot about "speed." When ads and reviews talk about how fast an HDTV is, they're referring to the display's refresh rate, or how often it changes the picture. Television and movies don't show actual motion so much as dozens and hundreds of frames per second, like a reel of film or a huge flipbook. The faster the HDTV, the more frames it displays per second.

So the faster the refresh rate, the better the HDTV, right? More frames look smoother, right? In theory: yes. In marketing: very yes. In practice: not so much.

Pulldown and the Film-Video DanceTo answer these questions, you have to understand two important things about video. First, you can't add detail beyond what is already in the source footage. Second, the source footage is never greater than 60Hz. When you watch a movie on Blu-ray, it's a 1080p picture at 60Hz. The disc displays 60 interlaced or 30 progressive frames at 1,920-by-1,080 resolution per second of video. For movies that were recorded on film, the original footage is actually 24 frames per second, upconverted to 30 frames through a process known as 2:3 pulldown. It distributes the source frames so they can be spread across 30 instead of 24 frames per second. Those frames are then interlaced (combined and shuffled) to 60 "frames" per second to match the 60Hz refresh rate of the vast majority of TVs you can buy today. In the case of 1080p60 televisions, the frames are pulled down to 60 full frames per second, and both the players and HDTVs outright skip any interlacing step.

This is a time-honored tradition, because American TVs have displayed 30 (actually, 29.97) frames per second and functioned at 60Hz since time immemorial. It's not really a problem, because between interlacing and frame pulldown, the process doesn't attempt to add information to the picture. It's simply converting it to function on the TV, because it wouldn't work otherwise. 1080p60 is the current high-end standard for HDTVs, and no commercial media exceeds that resolution or frame rate. In fact, many movies on Blu-ray even turn the frame rate down and display 1080p24, or 1,920-by-1080 video at 24 frames per second, to make the footage look as close to film as possible. The various refresh rate-increasing technologies on HDTVs destroy that effect.

Higher Refresh RatesOnce an HDTV's refresh rate goes above the rate of the content you're watching, it starts performing tricks to produce a higher frame rate. It interpolates new frames between the frames transmitted to the display at 60 frames per second (or processed into 60 frames per second from 24 frames per second for film footage, through the separate pulldown process), and the HDTV fills in the spaces by generating the best "middle" frames to stick in the cracks. These new frames are made by combining and processing the data of the frames surrounding them, generating the images the HDTV thinks it should draw between the images it's told to draw by the media. You're looking at more individual pictures as the screen draws them, but these pictures weren't on the Blu-ray disc or television signal that the screen is receiving; the HDTV is generating those additional pictures itself.

Good for GamesWhen flat-panel HDTVs were in their infancy, they suffered from motion blur. LCDs in particular, tended to display distinct blurriness during very fast movements because of "ghosting," or the afterimage left after the image on the screen has changed. LCD technology has progressed a great deal over the past several years, and now ghosting and motion blur have been all but eliminated.

Even without blur, you might notice choppiness or "tearing" (the effect of part of the image seeming to hang behind what's on the rest of the screen for a moment). This is especially noticeable in sports and video games, or any content that has a lot of fast, horizontal panning of the camera. For this, higher refresh rate modes can help.

What About 4K?Ultra high-definition television (UHD, or 4K) is still in its early stages, and while you can purchase a 4K HDTV you'll still have some problems getting any appreciable amount of media for it. The HDMI 2.0 standard has only just made 60 fps 4K video a consistent possibility for certain devices, and so much processing is already being used on simply displaying the much higher 3,840-by-2,160 resolution that interpolation and adding frames to make the action smoother hasn't been a major priority for HDTV manufacturers yet. Basically, if you want a 4K screen, don't expect it to break 60Hz for a while.

But Are Super-Fast Refresh Rates Worth It?Enhanced refresh rates can go too far. While 120Hz refresh rates seen on most midrange HDTVs can work well, don't expect to see any real performance improvement from 240Hz refresh rates or, for many plasmas, 600Hz. More importantly, you should know when to turn these enhanced refresh rates off, and watch with the "default" 60Hz or 24Hz film mode.

Refresh rates and motion-enhancing modes higher than 60Hz can produce a surreal effect when watching movies and television shows. The additional frames and "smoother" animation looks different from what we're used to with TV and movies, making the footage appear strangely fast. For any sort of content where you watch people interact naturally, like comedies or dramas, this can be unsettling and you should consider turning off the motion enhancing mode and force the screen to display the picture at 60Hz. However, for sports and video games, those added frames can help reduce stuttering and blur, and the action will be easier to track.

As a general rule, if what you're watching involves seeing real peoples' faces as they talk, disable the higher refresh rate so they don't look like creepy dolls (also known as the "soap opera effect"). If what you're watching involves seeing real people run into each other (sports), or fake people attacking each other in an artificial environment (video games), keep the higher refresh rate mode turned on (though set to "low" or "medium" if given the option, because the most zealous motion modes can still make the picture look unreal).

Remember these modes don't add any actual detail to the video, and you might want to disable them when watching every-day, non-action content. A 120Hz refresh rate can be beneficial for certain situations, but a higher refresh rate should not be considered a good reasons to spend more on an HDTV. For most television and movie watching, you'll probably want to keep the refresh rate set to 60Hz, anyway. Just keep the benefits in mind for sports and games, and don't feel the need to push past 120Hz. Anything higher really is more of a gimmick than a truly useful feature.

About the Author

Will Greenwald has been covering consumer technology for a decade, and has served on the editorial staffs of CNET.com, Sound & Vision, and Maximum PC. His work and analysis has been seen in GamePro, Tested.com, Geek.com, and several other publications. He currently covers consumer electronics in the PC Labs as the in-house home entertainment expert... See Full Bio

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